1. Field of the Invention
The present invention relates to an under bump metallurgic (UBM) layer, and more particularly to an improved structure of a UBM layer with less electromigration loss.
2. Description of the Related Art
For semiconductor industry, the manufacturing of integrated circuits comprises three main stages: fabrication of wafers, fabrication of integrated circuits and package of the integrated circuits. Dies are obtained by sawing wafers on which circuits are designed and manufactured. The dies connect external circuits via bonding pads thereon. The dies then are packaged so that the dies are protected from the influences of moisture, heat and noises. For the dies with packages, the package of the die acts as the medium for electrically connecting external circuits, such as printed circuit board (PCB) or other package substrate boards. Accordingly, the package of the dies and the integrated circuits is completed.
In order to connect the dies and the package substrate, wires and/or conductive bumps are often applied. In the Flip Chip Interconnect Technology (FCIT), an array of conductive bumps is formed on the boding pads of the dies. Then the dies are flipped so as to connect the conductive bumps to the contacts of the package substrate. Accordingly, the dies are electrically connected to the package substrate via the conductive bumps, and electrically connected to the external circuits via the internal circuits and the contacts of the package substrate.
FIG. 1 is a schematic cross sectional view of the structure of a prior art UBM layer. The passivation layer 104 is formed on the active surface 102 of the chip 100. The passivation layer 104 covers the active surface 102 of the chip 100. An opening 106 is formed in the passivation layer 104 and exposes the top surface 112 of the bonding pad 110, serving as a contact window for subsequent bumping process. The UBM layer 120 and a conductive bump are formed on the bonding pad 110 by the bumping process. The UBM layer 120 is formed between the top surface 112 of the bonding pad 110 and the conductive bump 130 for enhancing the adhesion between the bonding pad 110 and the conductive bump 130. The material of the conductive bump 130 can be a solder material such as Sn—Pb. The spherical bump can be formed by a reflow process.
It is noted that the UBM layer is formed substantially conformally to the structure of the opening 106, covering the top surface 112 of the bonding pad 110 and the surface surrounding the opening 106. During operation of the chip 100, large currents flow through the UBM layer 120, resulting in high current density at the region 108 close to the transmission line 114. Due to current crowding in the region 108 of the UBM layer 120, metal atoms diffuse along the electron flowing direction at the lattice boundary of this region 108. This phenomenon is called electromigration. Electromigration causes losses of metal atoms of the UBM layer 120. This electromigration phenomenon is more serious at the portion which is close to the transmission line 114 than the other portion which is away from the transmission line 114, which reduces the life time of the chip 100.